This invention relates in general to a method of treating quartz crystal resonators, and in particular to a method of reducing the aging and thermal hysteresis of quartz crystal resonators.
By aging as the term is used herein is meant the change in the resonance frequency of the quartz crystal resonator with time. Similarly, the term thermal hysteresis refers to the change in resonance frequency of the quartz crystal resonator due to a temperature change from the original temperature followed by a return to the original temperature.
As is well established in the art, and as used herein, the term quartz crystal resonator refers to a quartz plate that has been mounted in clips, the clips bonded to the quartz, and electrodes deposited on the quartz by plating in a vacuum system. Following the various processing steps, a contaminating layer is left on the resonator such as carbonaceous contaminants due to backstreaming from an oil diffusion pump, outgassing of the chamber, residues from electroplating, etc. In current practice, the quartz crystal resonator is then sealed in some housing or enclosure. Such sealing also causes contamination due to the flux used in solder sealing and the outgassing of the walls during heating. Even when stringent measures have been taken to eliminate the most obvious sources of contamination such as flux and oil vapors, it is well known in vacuum science that a clean surface exposed to air will absorb monolayers of contamination within a fraction of a second. Even in a vacuum of 10.sup..sup.-6 Torr, a monolayer of contamination can form in about one second. It is therefore virtually impossible to maintain atomically clean surfaces during processing.
The surface contamination can lead to aging and thermal hysteresis by several known mechanisms. To illustrate this point, one can consider a 5 megahertz fundamental mode resonator sealed hermetically in ultra high vacuum. If contamination equivalent to a single atomic layer of quartz desorbed from the surface of this resonator, the frequency would change by about 3 parts per million. If a monolayer desorbed from the enclosure walls as well, and if the desorbed contaminants remained in the enclosure in gaseous form, the pressure in the enclosure would rise to over 100 microns. Contamination also has a major influence on the electrode adhesion, and on the stresses at the electrode-quartz interface.
The aging caused by the contamination prevents the resonators from being used in modern communications and navigation systems which require highly stable resonators. For example, a stability of 2 parts in 10.sup.10 per week is required by one such system. Presently, the equipment required for processing resonators in an ultraclean, ultrahigh vacuum system in which the resonator is not exposed to contaminating atmospheres is extremely expensive and difficult to assemble. Moreover, even in the best of such systems, the resonator surfaces do adsorb some contamination during processing.
As is well known in radiation chemistry and photochemistry, when monomers are exposed to energetic radiation, the absorption of the radiation creates radicals and ions that can precipitate the formation of polymers. An example of radiation induced polymerization in vacuum is the troublesome buildup of contamination in electron microscopes due to the electron beam polymerization of pump oil vapors.